It is known in the semiconductor fabrication art to use an ion beam to dope semiconductor wafers with ion impurities. By scanning an ion beam across a wafer surface or moving the wafer through a stationary beam the wafer can be uniformly doped.
The angle at which an ion beam impacts a wafer surface (wafer tilt) is an important parameter in ion implantation of the wafer. Recent trends in semiconductor material processing require a greater range of wafer tilt capability, typically 0-60 degrees, while decreasing the variation of the ion impact angle across the wafer surface.
In a scanning ion beam system, electrostatic deflection plates produce a raster pattern of ion beam impingement on the wafer surface. One set of plates produces a rapid back and forth scan in one direction and a second set of plates provides beam deflection in an orthogonal direction. Such raster scanning results in impact angle variations of .+-.4.degree. of the central ray of the beam across a 200 mm wafer for a typical scanning ion beam geometry.
Methods have been proposed to reduce this impact angle variation. One proposal suggests using four sets of deflection plates, two horizontal and two vertical, and is referred to as a double deflection system. The beam is first deflected away from an initial trajectory and then, before striking the wafer, is deflected again to return to a direction parallel to its original, undeflected trajectory.
Use of a double deflection system with large wafer diameters requires deflection plates that are more widely spaced. This requires high deflection voltages that must be scanned and precisely synchronized with the scanning voltages applied to the first set of deflection plates. Another problem is that as the opening in the scan plates increases, electrostatic fringing fields become more difficult to control and become more susceptible to beam space charge effects.
Another known method of reducing tilt variations is to use a mechanically scanned, spinning disk wafer support. If the spin axis is parallel to the beam, no impact angle variations are present. Spinning disk supports have problems achieving control over impact angle while maintaining the necessary condition for an impact angle variation. One example of a prior art patent having a spinning workpiece support is U.S. Pat. No. 4,794,305 to Matsukawa.
Another more recent approach is to electrostatically scan the beam in one axis, and then use a highly indexed bending magnet to produce a parallel ribbon beam. The semiconductor wafer target is also scanned mechanically in a direction orthogonal to the ribbon beam to produce a uniform two dimensional implant. U.S. Pat. No. 4,276,477 to Enge, U.S. Pat. No. 4,687,936 to McIntyre et al. and U.S. Pat. No. 4,922,106 to Berrian et al. disclose such systems.
To define an ion beam used for implantation, one often refers to the median and range of impact angles for the beam against the workpiece. To help characterize the median and range, two orthogonal planes are defined; the parallelizing and the cross plane. The parallelizing plane is the plane seen from the top view of the lens in the '655 patent to Dykstra et al. and is also referred to as the scan plane since this is the plane in which the beam scans back and forth. The cross plane is seen as a section view through the lens that bisects the lens along the direction of beam travel.